1. Field of the Invention
The present invention relates to a gain control circuit to perform signal level control. In particular, the present invention relates to a gain control circuit to extract a wobble signal from the guide groove of a data-writable optical disc, and an optical recording and reproducing device using the gain control circuit.
2. Description of Related Art
In most of VCA (voltage control amplifier) circuits of the related art, the ratio between a mutual conductance gm1 and a mutual conductance gm2 is controlled with direct current by directly changing the in/out current ratio to perform gain control in a bipolar circuit using a Gilbert cell and a similar analog MOS circuit (for example, refer to Japanese Unexamined Patent Application Publication No. JP 10-276051).
When the gain control is performed by this type of circuit, in order to change the individual gm (the mutual conductance), the direct current passing therethrough is changed to control gm. Depending on the gain, the amount of the noise thereof may be varied and the frequency characteristics thereof may also be varied, thereby failing to hold uniform characteristics within a variable range.
That is, the changes in the mutual conductance gm1 and gm2 of the amplifier circuit lead to the change in output offset (generated by the VCA) per gain.
Japanese Unexamined Patent Application Publication No. JP 2005-45716 discloses an amplifier circuit and a semiconductor device having the amplifier circuit. The amplifier circuit includes a differential amplifier circuit and has the circuit configured to add the current changes in the positive phase and the negative phase and feed the resulting current to an additional resistor, while holding a variable current constant.